Radiation measuring instrument

ABSTRACT

A highly sensitive and compactly structured radiation measuring instrument for detecting ionizing radiation, in particular for measuring dose rates and contamination. 
     The laminar structure of the associated counter tube, using only a few, simple plastic parts (1, 8, 9) and a highly elastic counter wire (2), makes it possible to use the simplest manufacturing techniques. The service life of the counter tube construction, which is completely and permanently sealed and filled with gas, is expected to be more than 12 years. 
     The described counter tube can be adapted in optimal fashion to the available space in a pocket instrument if it is used in combination with a specialized high-voltage generator which is low in interference voltage and with a pulse evaluation circuit having a means of compensating for interference voltage.

This application corresponds to German patent DE No. 31 00 447 C2,issued Feb. 24, 1983.

The invention relates to a radiation measuring instrument for detectingionizing radiation.

The novel instrument should have high sensitivity to beta, gamma, X andneutron radiation, for instance, so that it will be suitable as acontamination measuring instrument for traces of weakly radioactivecontamination. Its simple, sturdy and compact construction should suitit for use as a universal radiation warning device. It should bepossible to manufacture and adjust the instrument according to theinvention with high accuracy, and to produce it simply andinexpensively.

BACKGROUND

Pocket-sized counter tube instruments for detecting radioactivity areknown. However, the miniature Geiger counters presently available arenot very sensitive and are therefore not suited to the detection ofslightly radioactive contamination in the environment. Specializedcontamination monitors are generally used for that purpose, beingequipped with special, highly sensitive counter tubes. The constructionof such instruments, however, is so expensive that there could be noquestion of their use except by professionals because of the high costinvolved. The basic situation thus described above in general will nowbe discussed in greater detail.

Some of the requirements to be made of a reliable radiation measuringinstrument are set forth in the DIN 6818 and DIN 44 801 standards. Itmust furthermore be noted that a hand instrument for measuringcontamination in the lowdose range must have a certain minimumsensitivity, so that a brief and/or slight increase in the counting rateabove the average background level occurring in the course of a scanningmovement can also in fact be recognized by the user of the device.

In practice, the only suitable contamination measuring devices have beenthose which respond to the natural background level of approximately 10microrem per hour with a counting rate of 1 s⁻¹ or more. At low countingrates, it is extremely difficult to detect variations in the countingrate directly, regardless of the manner in which the counting rate isregistered or displayed. When this is done using an indicatorinstrument, then even when the maximum permissible time constant of 4 s(DIN 44 801) is used, the reading accuracy at low counting rates isinsufficient. Optical or acoustical single-pulse registration producesno better results, because the human organism is not capable ofperceiving with sufficient accuracy stochastic events which elapseslowly.

From the above discussion it will be understood that the low-voltagecounter tubes having a noble-gas/halogen atmosphere presently availableand of standard design, and in particular the miniature versions, cannotbe considered appropriate for the construction of an efficient radiationmeasuring device suitable for measuring contamination, because thesensitivity of such standard detectors is insufficient for reliablydetecting weak radiation.

In recognition of this fact, the use of proportional counter tubes oflarge surface area was advocated at quite an early date. Proportionalcounter tubes are known which have been optimized, at relatively greatexpense, in the direction of maximum possible sensitivity and which aredesigned as measuring instruments for alpha and beta radiation, beingparticularly in the form of liquid flow counters having thin-walledwindows. Window-type counter tubes of this kind must, because of theirconstruction, all be operated with a constant high voltage of severalkilovolts, and therein lies one of the primary difficulties in theminiaturization of highly sensitive radiation measuring instruments.

Portable embodiments having a built-in, large-surface proportionalcounter tube are also known, which are commercially available asworkplace monitors in order to satisfy the requirements of Article 64 ofthe Radiation Protection Regulations of the Federal Republic of Germany.Given the considerable expense of these instruments, their structuralvolume of more than 2 dm³ and a weight of more than 1 kg, such devicesare used practically only in connection with the handling of radioactivesubstances requiring official permission according to Article 3 of theRadiation Protection Regulations of the Federal Republic of Germany.Universally useable pocket instruments having large-surface proportionalcounter tubes have not heretofore been known. The only pocketinstruments known for this purpose have been those with Geiger-Millercounter tubes, particularly counter tubes of the low-voltage type and ofclassic design.

THE INVENTION

It is the object of the invention to disclose a reliably functioningradiation measuring instrument usable as a radiation-protectiondosimeter, which although of small dimensions and having low currentconsumption has a high response capacity for weak radiation, and theconstruction of which is so simple and sturdy that the instrument can bemade available to a wide range of users.

Briefly the counter tube is made up substantially of only a few--thatis, two or three--simple plastic parts laminated together and a highlyelastic counter wire, and that associated with the counter-tubeconstruction, directly adjacent, are a low-interference high-voltagegenerator (which operates at low current consumption) for generating thesupply voltage for the counter tube, which operates in the proportionalrange, and a pulse evaluation circuit having an (adjustable) means ofinterference-voltage compensation.

As a result of the combination of the three characteristics according tothe invention, it is possible, using simple manufacturing techniques, toconstruct a reliably functioning radiation measuring instrument as apocket device and to equip it with a highly sensitive large-surfaceproportional counter tube although the latter requires a high voltage ofseveral kilovolts. The counter tube, the high-voltage generator and thepulse evaluation means, in particular, are disposed quite compactlyadjacent to one another in the exemplary embodiment which will bedescribed below, without taking expensive measures to provide shielding,and are accommodated together in a plastic housing of small dimensions.

A problem in such arrangements is the attainment of a sufficientinterference-voltage interval between the pulses to be picked up at thecounter wire and the interference pulses from the high-voltage generatorinjected into the evaluation circuit via stray capacitances. While inconventional counter tubes of the Geiger-Miller type, in particular inlow-voltage halogen counter tubes, the level of the emitted pulses isapproximately 10⁻² to 10⁻¹ of the supply voltage, these conditions aresubstantially less favorable in counter tubes operated in theproportional region. In that case, a peak pulse voltage must be expectedwhich is in the range from 10⁻⁶ to 10⁻⁴ of the supply voltage.

For safety reasons, it is required that the interference-voltageinterval in the signal processing be at least 20 dB below the smallestuseful signal to be taken into consideration in the evaluation; thismeans that in constructing instruments having proportional countertubes, interference-voltage intervals in the range from -140 dB to -100dB, with respect to the peak voltage of the high-voltage generator, mustbe achieved. The working frequency then is conventionally a fewkilohertz. These unfavorable conditions represent the primary reason whyproportional counter tubes operating with high voltage could notpreviously be incorporated into instruments of genuinely pocket size.

THE DRAWINGS

The realization of the three characteristics according to the inventionwill now be described in greater detail. One possible form of embodimentof the radiation measuring instrument according to the invention isshown in the drawings. Shown are:

FIG. 1, the laminated construction of the counter tube, seen in aperspective illustration;

FIG. 2, a section taken through the front holding device for the counterwire;

FIG. 3, a section taken through the rear holding device for the counterwire;

FIG. 4, a cross section taken through the counter tube;

FIG. 5, the distribution of the response sensitivity over the crosssection of the counter tube;

FIG. 6, the characteristic of the counter tube;

FIG. 7, a block circuit diagram of the high-voltage generator and of theinterference-voltage compensation means:

FIG. 8, a perspective illustration similar to FIG. 1, of the alternateembodiment of claims 3, 13, and 14.

DETAILED DESCRIPTION

All the above-mentioned requirements are satisfied in accordance withthe invention by means of a counter-tube construction in which thecounter tube substantially comprises three plate-like or foil-likeplastic parts, for instance laminated by means of an adhesive, and onecounter wire. FIG. 1 shows how these elements are combined in alaminated design. Specifically shown are: a window 8 in the form of asimple, perhaps rectangular, blank of a thin-walled,glass-fiber-reinforced metal/plastic laminate; a frame-like middle part1, for instance in the form of a plastic plate provided with a centralcutout; a base 9 in the form of a simple, perhaps rectangular, blank ofsome arbitrary metal/plastic laminate; and a metal wire 2 of highelasticity which is fastened in the middle part 1. FIG. 1 further showstwo insulating sleeves 3 of polytetrafluoroethylene (Teflon), a tubularrivet 4 for shielding the cable connection, a coaxial cable 5 forconducting the pulses and two filler fittings 10 of copper tubing.

The exemplary instrument shown in the drawings has a plastic housing theouter dimensions of 155×72×42 mm. The outer dimensions of the frame-likemiddle part 1 are 127×60 mm. The wall thickness W is 10 mm. The countertube structure completely fills the flat side of the housing.

In order to fabricate the window 8 and the base 9, copper-linedglass-fiber/epoxy laminate can be used, of a quality such as isavailable on the market as the basic material for printed circuits, downto surface densities of 50 mg/cm². The window and the base can be gluedto the frame-like middle part in such a way that each of the metallayers faces inward. The metal coatings serve as cathode surfaces.

The counter wire 2 used as an anode is fastened precisely in the centerbetween the cathode surfaces. Both the counter wire itself and theaccessory materials serving to fasten it can be effectively shieldedwith simple means because of the laminated design.

In high-quantity production, it may be efficacious to fabricate themiddle part 1 and the base 9 in one piece, for instance by injectionmolding, making a subcomponent 1+9 (FIG. 8). On the base of the plastichalf-shell thus created, a metal coating is made by lamination or byvapor-depositing in a vacuum. Otherwise, this variant does not differfrom the description given earlier.

In accordance with the invention, either polyvinyl chloride,polyvinylidene chloride, polyvinylidene fluoride or a mixed polymer ofthese components, free of softening agents, is used for the middle part1 of the above-described construction. These materials lend themselveswell to both chip-producing and thermoplastic processing, and whenepoxyresin adhesives are used, they result in good bonds with theabove-mentioned copper-lined plates. These materials furthermore havethe particular advantage, because of their highly polar molecularstructure, that the nonpolar counter gas to be described belowpenetrates them to only a very limited extent.

However, there is considerable prejudice against the use of plasticmaterials in the construction of counter tubes having a sealed gaschamber. The primary reason is that experience has shown that suchmaterials, over a rather long period, give off residual gases and arefurthermore not usually entirely impermeable to the counter gas.

There has been some experience with polymethylmethacrylate (Plexiglas)and polyethylene materials, to the extent that these materials have beenused in the construction of recoil proton counters. Whenever suchconstructions have been provided with a sealed gas chamber for the sakeof longer life, they are usually additionally sealed hermetically byenclosures made of thin metal sheets or metal foils.

The use of polymethylmethacrylate (Plexiglas) in counter tubes having asealed gas chamber is problematical, because this material producesparticularly high quantities of residual gases, and these gases arepredominantly made up of water vapor. Thus within a short time, watervapor partial pressures of more than 1 torr can arise within the countertube. Yet just such molecules, like H₂ O and O₂, which can form slightlynegative ions, present substantial interference even in tracequantities. Such a phenomenon would have particular impact on thecounter tube construction according to the invention, because thegeometry of the electrical field in the invention is less favorable thanin the conventional design, which has a radially symmetrical field.

Nor can the use of polyethylene, polypropylene or similarly structuredpolymers be considered possible for the construction according to theinvention, because such materials can be processed satisfactorily onlywith adhesive systems which contain solvents. Aside from the pooradhesion to metal surfaces in such a case, a very pronounced emission ofgas from the glued joint would then have to be expected. Because of theclose chemical relationship between these last-named polymers and thecounter gas to be described below, it must furthermore be expected thatthe gas will be highly soluble in the wall material. The coefficient ofthermal expansion of the polyethylene is approximately twice that of thematerial according to the invention. This material characteristic hasparticular significance for the construction described herein.

Nor can the material preferred for vacuum-type applications,polytetrafluoroetylene (Teflon), or the chemically closely relatedfluorohydrocarbons be used, because of the particular difficultiesassociated with adhesion in producing the middle or lower part of thestructure.

When polyvinyl chloride or similar materials are used, the use of copperin the interior of the counter tube offers a particular advantage: Thepartially oxidized surface of this metal chemically binds any slightresidual gas quantities which may arise, especially the primaryinterference factors H₂ O and O₂ and perhaps HCl or HF as well, veryfirmly and thus renders them harmless. This mechanism is important, ifdespite the use of plastic materials it is necessary to attain longservice life on the part of the counter tubes.

FIGS. 2 and 3, in a horizontal section seen on an enlarged scale, showthe accessory means with which the counter wire 2 is fastened within themiddle part 1. A soldered sheath 6 is located at the front end of thecounter wire and serves to connect the counter wire 2 to the coaxialcable 5. A small tubular rivet 7 serves as the rear termination of thecounter wire 2. The counter wire 2 is soldered to the metal parts 6 and7 with a specific initial stress. The two fastening devices shown inFIGS. 2 and 3 for the counter wire are sealed off by being plugged withepoxy resin.

Previously, it was generally the practice to maintain the mechanicaltension of the counter wire permanently by means of a spiral springdisposed in the counter tube. If in order to save space or for the sakeof economical manufacture a tension spring must be dispensed with, theneven greater demands are made on the material properties of the counterwire, so that this wire will remain under tension under all operatingconditions.

A further embodiment of the invention offers a satisfactory solution tothis problem, by providing that in counter tubes made up of plasticmaterials without spiral springs, alloys are used which unite inthemselves the characteristics of a high heat-expansion coefficient, alow modulus of elasticity, a wide range of fully-elastic deformability,high tensile strength and good solderability. According to theinvention, a hardened gold-silver-copper alloy having a composition of70% Au, 20% Ag and 10% Cu or else the hardened resistance alloy,manganin, having the short formula, CuMn12Ni, according to DIN 17 471can be used in counter tube constructions of plastic.

The conventional materials for counter wires, such as tungsten,platinum, chromic iron and others, fail in constructions of plastic,because in the course of temperature cycles either the wire tears or anirreversible deformation of the wire or of the middle part or lower partserving to fasten it occurs. Good solderability is likewise advantaqeousin constructions of plastic materials.

As the counter gas, highly-purified n-butane or highly-purifiediso-butane is preferably introduced into the gas chamber of the countertube. These gases are inert over long periods in the presence of all thematerials which are in contact with the gas chamber, in particular inthe presence of polyvinyl chloride and chemically similar polymers,cured epoxy resins, copper, tetrafluoroethene, the gold-silver-copperalloy mentioned above, the resistance alloy, manga-nin, and theflux-free soldering tin used in soldering. The high molecular weight ofthe proposed gas furthermore has a favorable effect in terms of lowerspeeds of diffusion. Because of the high proportion of hydrogen atoms inthe butane molecule, a special feature is also the possibility ofdetecting fast neutrons with the counter tube.

FIG. 4 shows a cross section taken through the counter tubeconstruction. In the sample instrument on which the drawings are based,the internal height h of the gas chamber is 8 mm and the total height Hof the structure is 10 mm.

FIG. 5 shows the distribution of response sensitivity of the countertube over the cross section of FIG. 4. Measurement was performed withthe mixed radiation of natural uranium, screened down to a 5 mm width.The supply voltage of the counter tube for this measurement is 3200 V,and the discriminator threshold is 100 mV. The effectively useful widthof the counter tube, measured as a half-width value HWB, amounts toapproximately 2.5 cm.

FIG. 6 shows the complete characteristic curve of the preferred form ofthe counter tube, which is the form described above. The characteristiccurve applies to the radiation of natural uranium distributed uniformlyover the total window surface area. The discriminator threshold is 100mV. In the vicinity of the plateau, a constant counting rate is obtainedover a range of 800 V. In aging experiments performed at an elevatedtemperature, which was the equivalent of an elapse of three years, nosubstantial changes in the counter tube characteristic could beascertained.

Further data for the sample counter tube in the preferred form ofembodiment:

Background level (without shielding): ca. 1.2 pulses per second.

Counting rate at a dose rate of 1 mR/h: ca. 120 pulses per second.

In summary, it can be said that the construction of the counter tubeaccording to the invention satisfies quite various criteria. Althoughthe window surface area is large, a low structural height is attained,so that the counter tube and the electronics can be accommodated aboveone another in a flat housing. For the window material, a material isused which on the one hand has good permeability to beta radiationbecause of its low surface density and furthermore, because of itscomposition, has a low wave-length dependency in the absorption ofphoton radiation, yet on the other hand has sufficient mechanicalstability that the window need not be protected against destruction byany specialized accessory structures such as protective screens or thelike. Without such accessory structures, the decontamination of themeasuring instrument becomes much easier.

With a view to the sensitivity to gamma radiation and also the long-termstability of the counter tube characteristic, the fact that the interiorof the counter tube is to a large extent defined by metal walls has afavorable effect. Walls of plastic materials are relatively thick, sothat gas diffusion processes directed either inward or outward remainwithin harmless limits.

Finally, the laminated structure also takes appropriate consideration ofthe above-discussed problem of the interference-voltage interval. Giventhe illustrated shaping of the cathode faces, good shielding is providedfor the counter wire itself, for its holders at either side and for thecable connection for the pickup of the pulses.

All the parts of the counter tube construction are distinguished bysimplicity of shape and are available on the market as ready-made parts.It is thus possible to produce the counter tubes economically and at afavorable cost. The construction is sturdy and is not vulnerable to themechanical stresses to which a universal radiation warning device may beexposed.

If the counter tubes must be used in the range of relatively high doserates, and if the conventional service life limit of from 10¹⁰ to 10¹²pulses is attained in counter tubes filled with gas which irreversiblydecomposes, then in accordance with a further embodiment of theinvention a particular purification principle can be applied: Byintroducing absorbents such as activated carbon, silica gel, aluminumoxide, magnesium oxide or the like into the sealed gas chamber, theharmful effect of the products of decomposition formed in thedischarging process can be reduced, and the service life of the countertubes exposed to strong radiation can be increased.

Further characteristics of the invention relate to the embodiment of theelectronic components. In terms of the electronics, special conditionsapply to a radiation measuring instrument of pocket size. Conventionalcircuit techniques cannot be used without adaptation, because thespatial closeness of the high-voltage generation and the pulseevaluation to one another leads to the above-discussed problems ofinterference voltage. In a simple pocket device, a complete shielding ofthe individual components in separate metal housings is not possible,because of the additional space required and the increased weight. Inpocket devices, not only is a specialized embodiment of the counter tubeconstruction required, for instance in the form of a laminatedstructure, but a high-voltage generator which is particularly low ininterference voltage is necessary; it is also advantageous to provide aseparate, adjustable means of compensating for interference voltage.

FIG. 7 provides a block circuit diagram for the high-voltage generatoraccordinq to the invention and for the associated means ofinterference-voltage compensation acting upon the pulse evaluationcircuit. FIG. 7 is explained as follows:

A push-pull sine wave converter GTSW is triggered continuously by aclock generator TG. The operating voltage is applied to the push-pullsine wave converter, however, only when the duty factor regulator TSTRgenerates a corresponding enabling signal. The enabling signals aregenerated whenever the counter tube operating voltage has fallen below apredetermined value. This is ascertained by the comparison of an actualsignal, picked up from the second stage A2 of the cascade multiplierKSKV via a voltage divider RV, P1, with a reference voltage REF.

The high voltage HSP is drawn from the topmost stage A10 of the cascademultiplier via a filter chain RS, CS. A low-resistance potentiometer P2is incorporated at the bottom of the filter chain RS, CS, by way ofwhich a predetermined component of the signal generated by the push-pullsine wave converter is coupled in via a compensating resistor RK inorder to compensate for interference voltage.

The purpose of compensating for interference voltage is not to attain acomplete smoothing of the high voltage, but rather to balance thepotentiometer P2 such that the maximum possible interference-voltageinterval is provided in the circuitry intended for the amplification andevaluation of the counter tube pulses. This is the case, for instance,if the interference-voltage portion primarily caused by the componentsof the cascade multiplier KSKV and injected into the evaluation circuitis neutralized, in a correctly phased manner and with the requiredamplitude, by means of the compensation signal which is superimposed onthe high voltage and conducted via the counter tube capacitance to theevaluation circuit. The amplification and evaluation of the pulses maybe accomplished with any conventional means.

In accordance with the invention, a substantial reduction in theinterference radiation emitted by the high-voltage generator is attainedin that the direct-voltage converter is designed not as a blockingconverter in the conventional manner, but rather as a push-pull sinewave converter generating signals having a low harmonic content. This isan unusual provision for a battery-operated device, because a converterof this kind has a relatively high current consumption, even with a lowsecondary load. However, this current consumption is reduced by morethan a factor of 10 in accordance with a further provision of theinvention, by designing the regulator for keeping the high voltageconstant as a duty factor regulator. In accordance with the controlsignals of the duty factor regulator, the push-pull sine wave converterneed not function continuously, but is switched on and off periodicallyinstead, with a duty factor such as is required for maintaining thecounter tube operating voltage. The converter can be switched on and offwith high efficiency if the converter is triggered by a clock generatorhaving a doubled working frequency.

A further distinction of the invention is that the improvement in theratio of the useful signal to the interference signal is attained inthat a compensation signal which can be adjusted in its amplitude isinjected into the filter chain of the high-voltage generator; thecompensation signal can be drawn from the push-pull sine wave converter.A compensation of this type can be optimally adjusted only if theinterference siqnal to be suppressed is as low in harmonics as is thecase with the push-pull sine wave converter, and if theinterference-voltage component is accordingly substantially identical infrequency and phase relation with the signal emitted by the push-pullsine wave converter.

The high-voltage generator of the sample instrument is made up of CMOSgates and standard operational amplifiers, and at input voltages of 5 to10 V it produces a high voltage, stabilized to within ±1%, of 3200 V.The current consumption at maximum is 3 mA. After the instrument hasbeen switched on, the high voltage is stable at 3200 V within 2 seconds.

In connection with the described invention, a particularly simple methodfor filling the counter tube with counter gas is also disclosed. Thefilling of the counter tube is effected in connection with a specializedflushing and aging process, which represents the foundation for greatlong-term stability of the counter tube characteristic and isdistinguished by a low labor cost. The fill pressure used isapproximately equivalent to atmospheric pressure.

Preferably all the counter tubes of one production batch, after beingglued, are connected in a series with one another via the fill fittings10 provided at the inlet and outlet sides; at an elevated temperature,they are first flushed for a sufficiently long period with dried air andthen with industrially pure argon, and finally with highly purifiedbutane at a slow flow rate and are thereby aged. After the flushing andaging process, the counter tubes are removed, one after another, withoutinterrupting the circulation of gas. As the individual counter tubes areremoved, the fill fittings 10 (FIG. 1), which are of thin copper, arepressed together and soldered.

I claim:
 1. A pocket-size radiation measuring instrument for detectingionizing beta, gamma, and neutron radiaion, which as its detector hasalarge-surface window-type counter tube suitable for measuringcontamination with a completely and permanently sealed gas chamber, aswell as a counter gas in said gas chamber, an anode and a cathode, ahigh voltage (multiple kilovolt) generator (HSP) connected to said anodeand cathode,wherein, in accordance with the invention, the counter tubestructure substantially comprises no more than three simple plasticparts (1, 8, 9), namely a substantially planar window (8), asubstantially planar base (9), and a frame-like hollow middle part (1),sandwiched between and sealed at its periphery to said window and base(9), at least one of said window and said base having a metal coatingthereon which serves as the cathode for said counter tube, and a highlyelastic counter wire (2) supported under tension within said hollowmiddle part (1) and serving as the anode for said counter tube.
 2. Aradiation measuring instrument as defined by claim 1, characterized inthat the counter tube substantially comprises three plate-like orfoil-like plastic parts bonded together by adhesive or by weldingtechniques and a counter wire, more specifically a window (8) in theform of a blank of simple shape cut from a thin-walled,glass-fiber-reinforced metal/plastic laminate, a frame-like middle part(1) in the form of a plastic plate provided with a central cutout, abase (9) in the form of a simply shaped blank of some arbitrarymetal/plastic laminate and a thin metal wire (2) having high elasticitywhich is fastened in the middle part (1).
 3. A radiation measuringinstrument as defined by claim 2, characterized in that the plasticparts defining the gas chamber of the counter tube are made exclusivelyof the materials, free of softening agents, of polyvinyl chloride,polyvinylidene chloride, polyvinylidene fluoride or a mixed polymer ofthese components, that the adhesive is a two-component epoxy resin, thatthe metal coating is of copper, and that the filling gas comprises anonpolar gas of the aliphatic series.
 4. A radiation measuringinstrument as defined by claim 2, characterized in thatabsorbentmaterials, which are inert in the presence of the counter gas, selectedfrom the group consisting of activated carbon, silica gel, aluminumoxide, and magnesium oxide, are accomodated inside the sealed gaschamber for the absorption of the products of decomposition produced byelectrical discharges between said anode and cathode.
 5. A radiationmeasuring instrument as defined by claim 1, characterized in that thecounter tube structure substantially comprises two simple plastic parts,bonded together by adhesive or by welding techniques, and a counterwire, more specifically a window (8) in the form of a blank of simpleshape cut from a thin-walled, glass-fiber-reinforced metal/plasticlaminate, a flat, plastic lower part having a flat base and an upperperipheral rim (1+9) produced by injection molding or deep drawing, tothe base of which a metal coating is applied, and a thin metal wire (2)having high elasticity which is fastened in the lower part.
 6. Aradiation measuring instrument as defined by claim 1 characterized inthat the material comprising the counter wire (2) is a hardenedgold-silver-copper alloy having a composition of 70% Au, 20% Ag and 10%Cu or the hardened resistance alloy, manganin, having the short formulaof CuMn12Ni as defined by DIN 17
 471. 7. A counter tube for detectingionizing radiation, which comprises essentially three plastic countertube elements, namely a window (8), a middle part (1), and asubstantially planar base (9), and an elastic counter wire (2),saidwindow and said base each having a conductive layer on the surfacefacing the other, wherein, in accordance with the invention, said middlepart (1) comprises a material selected from the group consisting ofpolyvinylchloride, polyvinylidene fluoride, and a mixed polymer ofpolyvinylchloride and polyvinylidene fluoride; said conductive surfacelayer of said window (8) and base (9) comprises copper; and said window(8), middle part (1), and base (9) are hermetically sealed together toform a gas chamber, and said gas chamber is filled with a butane countergas.
 8. A counter tube as defined by claim 7, whereinthe window (8) andbase (9) are formed as simple rectangular blanks of a thin-walled,glass-fiber-reinforced copper/plastic laminate, and said middle part (1)is formed as a frame-like plastic plate provided with a central cutout.9. A counter tube as defined in claim 7, characterized in that themiddle part (1) and the base (9) are formed as one integral piece byinjection molding.
 10. A counter tube as defined in claim 7,characterized in that the middle part (1) and the base (9) are formed asone integral piece by extrusion.
 11. A counter tube as defined in claim7, wherein said sealed gas chamber contains, in addition to the countergas, an absorber material for absorbing breakdown products resultingfrom electric discharges in said counter gas.
 12. A counter tube asdefined in claim 11, wherein said absorber is a material selected fromthe group consisting of activated charcoal, silica gel, aluminum oxide,and magnesium oxide.
 13. A counter tube as defined in claim 7, whereinsaid counter wire (2) and said conductive surface layer serve as theanode and the cathode, respectively, of the counter tube and are adaptedfor connection to a high-voltage supply.
 14. A counter tube as definedin claim 7, wherein said counter wire (2) is stretched across saidmiddle part (1) substantially parallel to said window (8).